From this month's Smithsonian, part of the text accompanying a c. 1900 of a postman with a toddler in his mail sack:

In 1914, a 2-year old boy was sent via Parcel Post from Oklahoma to Kansas (postage paid: 18 cents), and the service delivered at least two other children before it officially barred the mailing of human beings later that year.

Um, what!?!? As those posters at the post office say SOME THINGS WERE NEVER MEANT TO BE MAILED.